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Stoichiometry cheat sheet

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Stoichiometry cheat sheet

  1. 1. High School Chemistry - Core Concept Cheat Sheet 12: Stoichiometry Key Stoichiometry Terms Limiting Reactants  Stoichiometry: Using the mole ratio in the balanced equation and information about one compound to find information about another in the reaction.  Dimensional Analysis: Method of converting units by multiplying by ratio of equalities.  Molar Mass: Sum of all the atomic masses (from the periodic table) in the compound.  Solute: Substance being dissolved in a homogeneous mixture (solution).  Concentration: Measure of how much solute is dissolved in how much solution.  Molarity (M): A concentration unit in moles per unit liter.  Standard Temperature and Pressure (STP): 1 atm (or 101.3 kPa) and 273 K (0).  Molar volume of a Gas: 1 mole of any gas at STP is 22.4 liters.  Limiting Reactant: Reactant that stops the reaction by running out first.  Actual Yield: The amount actually produced in the lab.  Theoretical Yield: Amount that should be theoretically produced based on stoichiometric calculations.  Percent Yield: Compares the actual yield to the theoretical yield (their ratio with %).  Once a reactant has run out, the reaction will stop.  Do stoichiometry for each given reactant quantity to the same product each time. Choose the calculation that gives the smallest amount of product.  The reactant that produced the smallest amount of product is the limiting reactant.  Steps to determine the limiting reactant: grams reactant  moles reactant  moles product  grams product. Alternatively, use the m/c method below. Simple Limiting Reagent Finder Mnemonic: Find the moles of each reactant and divide the moles of each reactant by its coefficient. The reactant with the smallest number is the limiting reagent = “Smallest m/c is Limited”. Chemical Equations  Coefficients give the mole ratios. Example: 2 H2 (g) + O2 (g)  2 H2O (l) 2 moles of H2 react with 1 mole of O2 and produces 2 moles of H2O. Dimensional Analysis in Stoichiometry Equalities used during dimensional analysis for stoichiometry:  Mole ratio in balanced equation: Use to convert between moles of different compounds in the balanced equation.  Molar mass: Used to convert between grams and moles  Concentration: Used to convert between moles and liters of a solution.  Molar volume of a gas: Used to convert between moles and liters of a gas at STP. KUDOS for Stoichiometric Problems Method to solve word problems:  K = Known. Identify all the known information (given quantities, balanced equation, etc.).  U = Unknown. What quantity and what compound does the problem ask for?  D = Definition. Identify the equalities that will be needed during dimensional analysis to convert from the known to the unknown.  O = Output. Perform the dimensional analysis.  S = Substantiation. Check for reasonableness, units and correct significant figures. Molarity Molarity – the concentration measure by the moles of solute per liter of solution. Percent Yield % yield  actual yield  100 theoretica l yield Example: If stoichiometry calculations tell you that a reaction should produce 10.5 g, but when you perform the lab you produce 8.7 g, what was the percent yield? % yield  8.7 g  100  82.9% 10.5 g Stoichiometry Examples All examples for the following reaction: 2 HCl (aq) + Mg (s)  H2 (g) + MgCl2 (aq) Mole-Mass example: 0.75 moles HCl react. How many grams Mg are needed? 0.75 mole HCl 1 mole Mg 24.31 g Mg = 9.1 g Mg 2 mole HCl 1 mole Mg Mass-Mass example: If 2.5 g Mg react, how many grams MgCl2 are produced? 2.5 g Mg 1 mole 1 mole 95.21 g Mg MgCl2 MgCl2 = 9.8 g MgCl2 24.31 g 1 mole 1 mole Mg Mg MgCl2 Mass-Liter example (solution): How many liters of 1.7M HCl are needed to react with 2.5 g Mg? 2.5g Mg 1 mole 2 mole 1 L HCl Mg HCl = 0.12 L HCl 24.31 g 1 mole 1.7 mole Mg Mg HCl Mass-Liter example (gas): How many liters of H2 are produced when 3.5 g Mg react? 3.5 g Mg 1 mole 1 mole 22.4 L Mg H2 H2 = 3.2 L H2 24.31 g 1 mole 1 mole Mg Mg H2 Limiting reactant: If 0.25 mole HCl reacts with 0.55 mole Mg, how many grams of MgCl2 are produced and what was the limiting reactant? 2HCl + Mg  MgCl2 + H2 0.25 mole HCl 1 mole 95.21 g = 12 g MgCl2 MgCl2 MgCl2 2 mole HCl 1 mole (less) MgCl2 95.21 g MgCl2 = 52 g MgCl2 1 mole MgCl2 12 g MgCl2 will be produced and HCl is the limiting reactant. How to Use This Cheat Sheet: These are the keys related to this topic. Try to read through it carefully twice then write it out from memory on a blank sheet of paper. Review it again before the exams. moles solute Molarity  L solution 0.55 mole Mg 1 mole MgCl2 1 mole Mg RapidLearningCenter.com ::  Rapid Learning Inc. :: All Rights Reserved

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